1. Field of the Invention
The present invention relates to an apparatus for detecting displacement information.
2. Description of the Related Art
A disk coated with a magnetic material is generally used as memory reproducing means for use with information processing apparatus, such as computers and the like. When signals (information) are magnetically written on the disk by a magnetic head, information for positioning the magnetic head must be previously written on the disk. Thus, pulses with a high frequency are first recorded in an unused area, mainly in the outermost periphery, of the disk by using an exclusive clock head separate from the head for writing information. Various angle division apparatus have been previously proposed in which the area on a disk is circumferentially divided into a plurality of portions on the basis of the pulses.
FIG. 1A is a schematic drawing illustrating a state where a disk is divided into a plurality of areas in the circumferential direction (angular direction). In FIG. 1A, reference numeral 1 denotes a disk to be divided. The drawing shows the state where the area on the disk 1 is divided into N equal areas. A portion between points a and b indicates one divided area on the disk. Although it is not always necessary to divide the disk at equal intervals, it is generally required to accurately divide a space of the disk at equal intervals, i.e., accurately divide the angle thereof.
Signals (information) are written on concentric tracks on the disk in each area. Positioning is thus performed by reading the positioning information (servo pattern) written at the top of each area. It is thus required to accurately divide the disk into areas without deviations between the respective concentric circles.
FIG. 1B is a schematic drawing showing a principal portion of a conventional angle division apparatus. In FIG. 1B, reference numeral 1 denotes a disk which is rotated by a spindle motor (not shown). Reference numeral 12 denotes a clock head for writing clock pulses with accurate time in an unused area of the disk 1 and reading the clock pulses therefrom. Reference numeral 1d denotes a clock pulse signal, and reference numeral 3 denotes a counter for counting the pulses generated from the clock head 12. Reference numeral 4 denotes a one-round pulse number memory for storing the number of pulses for one round of the disk 1. Reference numeral 5 denotes division number data indicating the number of divided areas for one revolution of the disk 1.
Reference numeral 6 denotes a divider for dividing the content of the one-round (one-revolution) pulse number memory 4 by the division number N of the division data 5 to determine the number of pulses per divided area. Reference numeral 7 denotes a divided pulse number memory for storing the number of pulses per divided area. Reference numeral 8 denotes a comparator for comparing the divided pulse number with the counter value to output division reference pulses for the divided areas.
Reference numeral 9 denotes a signal processing unit for writing and reading data through the magnetic head 11. Reference numeral 10 denotes a voice coil motor (referred to as "VCM" hereinafter) for driving the magnetic head 11 in the circumferential direction of the disk 1. The magnetic head 11 writes and reads data. In FIG. 1B, a fine line shows a pulse signal, and a thick line shows a data bus.
In FIG. 1B, the clock pulses with accurate time generated from a clock pulse circuit (not shown) are written, by the clock head 12, in an unused area of the disk 1 rotated by the spindle motor. The number of clock pulses for one revolution is previously determined, and the clock pulses are written while changing the frequency of the clock pulses until a correct number of pulses are written in.
The clock head 12 reads the written clock pulses, and the number of the clock pulses counted by the counter 3 is stored in the one-round pulse number memory 4. This value is divided by the division number of the division number data 5 and stored as the number of pulses per divided area in the divided pulse number memory 7.
The number of the pulses counted by the counter 3 is compared with the number of pulses per area by the comparator 8. The comparator 8 counts the pulses for one area and transmits as a reference pulse the number of pulses to the signal processing unit 9. At the same time, the comparator 8 sends a clear pulse to the counter 3 for making preparation for counting pulses from zero in a next area.
When receiving the division reference pulses, the signal processing unit 9 writes a signal for dividing the area on the disk 1 by using the magnetic head 11. The VCM 10 successively positions the magnetic head 11 at the predetermined intervals in the circumferential direction by a circumferential positioning device (not shown) to radially form position signals as division references on a concentric circle at equal angles by the magnetic head 11.
In the angle division apparatus shown in FIG. 1B, since the clock pulses are based on time, variations in the rotational speed affect directly the precision of angle division. In some cases, only during writing of the clock pulses, the rotational speed of the disk 1 is lowered for increasing the resolution by decreasing the effects of the variations in the rotational speed. This is the cause of interference with an increase in throughput. There is also the problem that defective products occur due to contact between the clock head and the disk.
Aside from the above angle division apparatus, a laser Doppler speedometer is conventionally used as an apparatus for precisely measuring the movement speed of a moving object without contact therewith. The laser Doppler speedometer applies a laser beam to the moving object and measures the moving speed thereof by employing the effect (Doppler effect) that the frequency of the light scattered from the moving object is shifted in proportion to the moving speed of the moving object, i.e, by detecting the Doppler signal.
The reference signal at the start position of measurement is sometimes used for detecting the information regarding movement of the moving object. In this case, an optical detector for detecting the Doppler signal and an optical detector for obtaining an index signal as a reference for the start position of measurement are provided.
In a conventional apparatus, the optical detector for detecting the Doppler signal and the optical detector for detecting the index signal are provided separately. There is thus the tendency that when the two optical detectors are provided opposite to the moving object, if the moving object is small, the whole apparatus is complicated.